Air conditioning with mixing duct

ABSTRACT

An air conditioning system and method of operating an air conditioning system wherein conditioned primary air is transmitted to a plenum chamber in a terminal unit, which terminal unit may be positioned for example, directly within a space to be conditioned or supported above a suspended ceiling in a space to be conditioned. A portion of the primary air is utilized to pass through nozzles and establish primary air flow in a mixing duct, which flow may induce and mix with a flow of secondary air to constitute supply air for discharge into a space to be conditioned. The supply air may comprise solely primary air or selectively variable volumes of primary air and secondary air, which secondary air may include selectively variable volumes of air recirculated from the space being conditioned and, if appropriate, air from the plenum chamber formed by a suspended ceiling in the space to be conditioned. Additionally, an air handling or terminal unit according to the invention may include a casing connected to a source of primary air for providing a constant flow of primary air and a selectively variable flow of primary air, means for selectively introducing and mixing a flow of secondary air with the constant flow of primary air, the secondary air comprising selectively volumes of recirculated air from the space and, if appropriate, volumes of air drawn from the plenum chamber defined by a suspended ceiling in the space to be conditioned and a control means for controlling the thermal capacity of supply air for the space, the supply air including the primary air from the constant flow and, selectively, volumes of additional or by-pass primary air and secondary air.

United States Patent Chandler [11 3,823,870 July 16,1974

[ AIR CONDITIONING WITH MIXING DUCT [75] Inventor: Robert B. Chandler, South Pasadena, Calif.

[73] Assignee: Kilpatrick & Company, Alhambra,

' Calif.

[22] Filed: Dec. 27, 1971 [21] Appl. No.: 212,593

Related US. Application Data [62] Division of Ser. No. 54,782, July 14, 1970, Pat. No.

521 user 236/49, 98/38 51 lnt.Cl F24f 3/08 [58] Field of Search 98/38, 40 o; 236/49 [56] References Cited UNITED STATES PATENTS 3,145,926 8/1964 ODay..... 236/13 3,610,522 l0/1971 Tutt 98/38 3,611,908 10/1971 Spoormaker 98/38 Primary Examiner-William E. Wayne r [57 ABSTRACT An air conditioning system and method of operating an air conditioning system wherein conditioned primary air is transmitted to a plenum chamber in a terminal unit, which terminal unit may be positioned for example, directly within a space to be conditioned or supported above a suspended ceiling in-a space to be conditioned. A portion of the, primary air is utilized to pass through nozzles and establish primary air flow in a mixing duct, which flow may induce and mix with a flow of secondary air to constitute supplyair for discharge into a space to be conditioned. The supply air may comprise solely primary air or selectively variable volumes of primary air and secondary air, which secondary air may include selectively variable volumes of air recirculated from the space being conditioned and, if appropriate, air from the plenum chamber formed by a suspended ceiling in the-space to be conditioned.

Additionally, an air handling or terminal unit according to the invention may include a casing connected to a source of primary air for providing a constant flow of primary air and a selectively variable flow of primary air, means for selectively introducing and mixing a flow of secondary air with the constant flow of primary air, the secondary air comprising selectivelyvolumes of recirculated air from the space and, if appropriate, volumes of air drawn from the plenum chamber defined by a suspended ceiling in the space to be conditioned and a control means for controlling the thermal capacity of supply air for the space, the supply air including the primary air from the constant flow and, selectively, volumes of additional or by-pass primary air and secondary air.

16 Claims, 14 Drawing Figures PATENTEB JUL 1 s 1924 saw 1 or 9 -FIG.

PATENTED 7 SHEEHHIF Q O. O O 7 6 5 f D\ w s 3 .0 C2 B\ 7 0 w 0 O O O 0 o 0 O O 0 w 9 8 7 6 5 4 3 2 m 0 m 1 4-0 or suPPu/Ammouceo FROM CONDITIONED SPACE (REbIR.

AIR) 86 OF SUPPLY AIR moucao FROM CEILING- SPACE or BY-PASS PRIMARY AIR m SUPPLY AIR FIG-.6

PAIENTEBJUL 1 a mu samsnfa FIG.

FIG. 9

PAIENIEB .IJL I 6 I974 SHEHBOFQ FIG. l'2.

Pmmzumw n SHEET 9 [1F 9 FIG.

FlG. l4-

AIR CONDITIONING WITH MIXING DUCT This is a division,,of application Ser. No. 54,782 filed July 14, 1970, now US. Pat. No. 3,720,258.

BACKGROUND OF THE INVENTION the effective temperature of air in the space be maintained within a small range. As thermal loads in the.

space vary, the thermal capacity of the air supplied to the space by the system must be varied in order to maintain the air temperature in the space within the allowable range. Variation in the thermal capacity of supply air can be accomplished either by delivering a constant volume of air to the space at a varying temperature, or be delivering a variable volume of air to the space at a constant temperature. l

' Delivery of a variable volume of air to a space is often unsatisfactory because the ordinarily used fixed air diffusers do not compensate for variations in the volume of supply air and as such, proper circulation of supply air in the space is seldom achieved. Diffusers which are capable of compensating for variations in volume of air flow are relatively expensive and often difficult to maintain/From the room circulation standpoint, therefore, it is desirable to provide a relatively constant volume of conditioned air to a space.

Among the various approaches to providing such a relatively constant volume of supply air to a space is the induced air method of air conditioning wherein a flow of primary air is utilized within an air handling unit to induce a flow of secondary air which combines with the primary air to provide a flow of supply air to the space to be conditioned. Thermal capacity of the supply air is ordinarily varied by causing the secondary air to pass over a heat exchanger containing an exchange medium which exchanges heat with the secondary air in response to the sensed thermal load in the space to be conditioned. The use of such a heat exchanger in such known induction type units, however, has caused the units to be relatively expensive. Further, their use requires the provision of a heat exchange medium at the unit which often necessitates connecting air handling units with central conditioning equipment through a system of pipes. Units not having such heat exchange equipment are known in the art, however, they have been limited in their capability to compensate for variations in thermal load and for other reasons often have been operationally unsatisfactory.

SUMMARY OF THE INVENTION It is the principal object of the invention, therefore, to provide an air conditioning system wherein a primary air system with induction type terminal units can be utilized, without the necessity for providing heat exwhich may include apparatus for conditioning and distributing a volume of primary air to a space tobe conditioned, a terminal unit in the space wherein at least a portion of the primary air is passed through nozzles to establish a flow of primary air in a mixing duct in the unit, which portion of primary air may induce a flow of secondary air in the unit and mixes with selectively variable volumes of the secondary air or by-pass primary air to constitute supply air for'the space to be conditioned, and a by-pass means, independent of the nozzles, for accommodating the passage of the by-pass primary air into themixing duct, the selective volumes of by-pass primary air and secondary air being'variable This object and others not enumerated are achieved I by the air conditioning system of the present invention in response to changes in the temperature of air in the space being conditioned.

The invention also includes a novel terminal unit, one embodiment of which may include a casing connected to a source of primary air for providing constant flow of primary air and a selectively variable flow of primary air, means for selectively introducing and mixing a flow of secondary air with the constant flow of primary air, the secondary air comprising selectively volumes of recirculated air from the space and, if appropriate, volumes of air drawn from the plenum chamber defined by a suspended ceiling in the space to be conditioned, and a control means responsive to variations in the temperature' of air in the space for controllingthe thermal capacity of supply air for the space, the supply air including the primary air from the constant .flow and, selectively, volumes of additional or by-pass primary air and secondary air.

The invention also includes a method of operating an air conditioning system for air conditioning a space which may includethe steps of conditioning primary air, transmitting the conditioned primary air to a terminal unit, passing a portion of the conditioned primary air through nozzle means to establish a flow of supply air into the space, mixing an additional volume of air with the portion of the conditioned primary air to constitute the supply air, the additional volume of air selectively comprising volumes of secondary air and by-pass primay air, the ratio of which volumes is selectively variable in response to changes in the thermal load conditions of the space. 1

BRIEF DESCRIPTION OF THE DRAWING FIG. 3 is a cross-sectional view through the plane 3-3 of FIG. 2;

FIG. 4 is a cross-sectional view through the plane 4-4 of FIG. 2; v

FIG. 5 is a cross-sectional view through-the plane 5-5 of FIG. 2;

FIG. 6 is'a graphical presentation of the operation of 'the embodiment of the terminal unit shownin FIGS.

an air conditioning system acyet another embodiment of a terminal unit according to the present invention;

FIG. 11 is a cross-sectional view through the plane lI-llofFIG. l;

FIG. 12 is a perspective view, partially cut away of still another embodiment of a terminal unit according to the present invention;

FIG. 13 isa cross-sectional view l313 of FIG. 12; and

through the plane FIG. 14 is a cross-sectional view through the plane l414 of'FIG. l2.

DETAILED DESCRIPTION Referring to FIG. 1, there is shown installed in a building 10 an air conditioning system ccording to the invention and designated generally by the reference numeral 12.

Air conditioning system 12 comprises a main primary air handling unit having an outside air fan 14 for providing ventilation'air to the system, a-filter section 16 which may be any of-the filter sections known to those having skill in the art, a cooling coil 17, a heating coil 18 and a main supply fan 20 which establishes a flow of primary air through the. system.

The dry bulb and wet bulb temperatures of primary air leaving the air handling unit are controlled by a suitable thermostat 22 which controls the operation of control valves 24 and 25 and thereby the flow of heat exchange media through the cooling and heating coil 17 and 18 respectively. In this regard, the temperature controls for system 12 may be suitable controls of any of the types generally known in the art.

Primary air is transmitted from the primary air handling unit to' spaces to be air conditioned through a main primary air duct 28 and floor runout ducts 30 which are shown mounted above the ceiling 31 of spaces 33 to be air conditioned. From run-out ducts 30,

Considering now the structure and operation'of terminal units 35, one embodiment of such a terminal unit according to the invention is shown in FIGS. 2-5 and designated generally by the reference numeral 35A. Terminal unit 35A includes a casing 37 having upper and transverse extending walls 39 and 41 longitudinally extending vertical side walls 42 and 43, and transversely extending vertical end walls 46 and 47. Formed in the lower portion of casing 37 is recessed channel 52 which is suitable for receiving a light fixture 53 therein.

Formed in end wall 47 is an opening 55 having a collar 56 to which may be secured air pipe 36 (FIG. 1) for.

providing unit 35A with primary air from run'out duct 30. Opening 55 communicates with a primary air plenum chamber 58 formed in casing 37 by the cooperation of upper wall 39, a vertically longitudinally extending side wall 61, and wall '47, a transversely extending plate 60 which defines the upper wall of recess 52, 'a transversely vertically extending wall 62, and a longitudinally vertical wall 64.

An opening 66 is formed in transverse wall 62 to communicate primary air plenum chamber 58 with a longitudinally extending duct 68 best seen in FIG. 4 defined by a transverse plate 70 which extends from side wall 42 across the major portion of casing 37 to a point generally above the vertical wall of recess 52, an upper duct wall 72 and side duct'walls 73 and 74. Plate 70 extends longitudinally from transverse wall 62 to end wall primary air is supplied to terminal units, designated generally by the reference numeral 35, through air pipes 36.

Primary air in terminals units 35 may be mixed with .secondary air, as is discussed below in detail, and supplied to spaces 33 so 'as to effect proper airconditioning and circulation. Air leaving spaces 33 and not recirculated as-secondary air as is discussed below, passes through suitable openings 38 in ceiling 31 to the space thereabove, and'thereafter, downwardly through return air shaft 40 to be re-conditioned in the primary air handling unit.

It can be seen, therefore, that system 12 functions byv conditioning primary air at a main air handling unit, transmitting the primary air to terminal units at the spaces to be conditioned, selectively mixing the primary air with secondary air as discussed below to provide supply air to spaces 33 at a thermal capacity proper for the prevailing thermal load, and returning air fromthe spaces to the main air handling unit through a return air shaft.

46 in a plane which is parallel to and spaced above transverse plate 60.

As is evident from FIG. 4, the transverse dimension of plate 60 corresponds to the width of recess 52. Between end wall 47'and transverse wall 62, however, plate 60 extends beyond recess 52 to side wall 42 (FIGS. 2 and 3), thereby to cooperate with side wall 42, upper wall 39, vertical walls 62 and 64 and end wall 47 to define a primary air by-pass plenum chamber 77.

cal wall 64. Shaft 81 is supported for longitudinal reciprocation by a suitable bearing 84 mounted centrally of a perforated cylindricalccage 86 which extends coaxially with shaft 81 from side wall 42 to opening 83. Shaft 81 is operably connected to a damper operator 88, which may be a pneumatic motor or the like, through a spring coupling 89so as to permit continued operation of damper operator 88 after the engagement of damger with opening 83 as is discussed below.

Formed in vertical wall 62. is an opening 91 which communicates primary air by-pass plenum 77 with a primary air space 93 above transverseplate 70, As is discussed below in detail, a flow of primary air is established through space v93 to the space to be conditioned under highcooling load conditions. Thespace defined by sidewall 42, transverse plate 70 and the side wall of recess-52 (FIG. 4) defines a passage through which a flow of air can be induced either from the conditioned space through an inlet opening 92 on through a dampered orifice 94 formed in side wall 42 of easing 37. Orifice 94 is selectively opened and closed by a pivoted damper 96, of conventional construction, which is operated between open and closed position by a camlinkage operator 98' (-FIG. 3) actuated by damper operator 88 through spring coupling 89. Damper 96 also operates to close passage 90' when it is in the fully open position (not shown) with respect to orifice 94.

Depending upon the range of thermal capacity of air to be supplied to a space to be conditioned, it may be desirable to provide. a heating means such as strip heater 99 over orifice 94 so that additional heat may be added to air entering the unit'through orifice 94. It is to be recognized, however, that such a heater may not be required, particularly in installations wherein terminal units 35 is recessed within a suspended ceiling and the ceiling contains air heated as a result of light load conditions and the like.

The space 100bounded by side wall 43, upper casing wall 39, side duct wall 74 and the side of recess 52 (FIG. 4) defines a passage throgh which a flow of air can pass from terminal unit 37 through outlet opening 101 to the space to be conditioned.

The flow path comprising space 90, the space 102 between plates 60 and 70, and space 100 defines a mixing duct wherein a flow of primary airinduces and mixes with a flow of secondary air to form supply air for discharge through outlet opening 101 into a space to be conditioned. A relatively constant flow of primary air for inducing a flow of secondary air through passage 90 is introducedto the mixing duct through a plurality of nozzles 104 (FIGS. 3 and. 5) which are mounted in transverse plate 70 and communicate primary air duct 68 with the space 102 between plates 60 and 70. In this I regard, nozzles 104 operate in the conventional manner by discharging primary air into space 102 at such a velocity as to induce a flow of secondary air to be carried along therewith.

Considering, therefore, the operation of terminal unit 35A, primary air is introduced to primary air plenum chamber 58 from air pipe 36 (FIG. 1) through opening 55. A constant flow of the primary air passes unrestrictedly from plenum chamber 58 through opening 66 into air duct 68 and thereafter through air nozzles 104 into the mixing duct. Under certain operating conditions,

the passage of primary air through nozzles 104 may induce and mix with a flow of secondary air from passage 90 which mixture is discharged through outlet opening 101 into a space to be conditioned, e.g., space 33of FIG. I.

The volumetric make-up of supply air being discharged through outlet opening 101 at any time is determined by the thermal requirements of the space being conditioned. Thus the supply air can be made-up of substantially all primary air during periods of maxi- 1 mum cooling requirements, -i.e., the volume of con- The thermal load requirements of the air in the space to be conditioned may be sensed by a thermostat such as thermostat 106 in FIG. 1. Changes in air tempera- .ture are sensed in the space to be conditioned and a resulting signal is sent to damper motor 88 to adjust-the make-up of supply air to accommodate for the change.

occurring in the thermal load of the room. Thus, when thermostat 106'senses such a rise in space temperature as to call for full cooling, damper operator 88 is actuated to fully withdraw conical damper 80 from opening 83 to allow conditioned by-pass primary air to flow from primary air plenum chamber 58 into by-pass plenum 77 through opening 91. Thereafter, the by-pass primary air flows over duct 68 and into space 100 where it is mixed with the constantly flowing primary air from nozzles 104 and discharges as supply air through'outlet opening 101. The relatively unrestricted flow of by-pass primary air into passage'l00 during full cooling causes the airpressure ,in passage 100 to be such as to virtually eliminate the induction of any secondary air from passage through nozzles 104.

As less cooling in the space is required, however,

conical damper 80 is modulated toward opening 83 by damper operator 88 so as to reduce the flow of by-pass primary air into plenum chamber 77 and ultimately passage 100. The throttling effect of damper 80 also introduces a pressure, drop into the by-pass primary air system thereby reducing the pressure of air in passage 100 so as to allow the flow of primary air through nozzles 104 to commence inducing a flow of secondary air from passage 90. At this stage of the operation, damper 96 is in the-closed position thereby providing that all secondary air induced by the flow of primary air through nozzles 104, isrecirculated air, i.e., air from the conditioned space entering passage 90 through inlet opening 92. t

The amount of secondary air induced by the flowing primary air at any time should be substantially equal to the reduction in flow of primary air from the full flow condition soas to maintain the volume of supply air substantially constant. The. thermal effect of such a volume of unconditioned air-which iszinduced from the space isto reduce the thermal cooling capacity of the supply air to'the space. Thus a reduction in space cooling load is compensated for by reducing the volume of by-pass primary air being supplied and correspondingly increasing the volume of return air being recirculated by induction so that the thermal cooling capacity of the supply air 'can be reduced without any substantial 4 change in'volume of the supplied air.

stantly flowing primary air plus a volume of by-pass primary air, and during periods of lesser cooling load, can

however, it is desirable for purposes of maintaining proper circulation, to provide a relatively constant volume of supply air to the space being conditioned, notwithstanding variations in the make-up of the supply air.

Continued reduction in the cooling requirement of the space causes a further'modulation of conical damper toward the closed position thereby progressively reducing the amount of conditioned primary air in the'supply air, and increasing the amount of recirculated air, thus reducing the thermal cooling capacity of the supply air. It should be recognizei however, that the supply of primary. air-to the space is never completely stopped because at all times during the operation of the system, primary air'flows through nozzles 104. V

During approximately the last ten percent of the ad-- Vance of conical damper 80 toward opening'83, pivoted damper 96 commences tofluncover orifice 94 thereby resulting in the air being induced in passage a both through inlet opening 92 and orifice 94. This 90 by the flow of primary air unit 35A is mounted in a ceiling space which contains warmer air than the environmental air in the space because of the mounting of lights or other heat generating equipment in the ceiling space. Specifically, by using this heatedair, the thermal cooling capacity of the supply air can be further reduced, and, depending upon the temperature of air in the ceiling space, a heating capability can even be achieved without a necessity for utilizing a supplemental source of heat. 7

A further reduction in the cooling requirement of the space causes damper operator 88 to further displace damper 96 in the clockwise direction so as to continue to open orifice 94 and to progressively cover space 90 until, at some point, all secondary air is being drawn into terminal unit- 35A through orifice 94. If further heating of the secondary air is required after space 90 is completely closed, the use of a heating means such as strip heater 99 becomes necessary. It has been found, however, that most commercial and domestic tional heating means.

It was noted above that even after the engagement of conical damger 80 with opening 83, damper operator 88 continues to operate damper 96'between partially and fully open positions. This is accomplished by attaching cam linkage operator 98 to the output shaft of damper operator 88 on the operator side of spring coupling 89. Thus, although shaft 81 is restrained against further displacement to the right as seen in FIG. 3 once conical damper 80 is engaged with opening 83, further operation of damper operator 88 causes the spring of spring coupling 89 to be compressed thereby allowing further displacement of cam-linkage operator 98.

The operation of terminal unit 35A may better be understood from a consideration of FIG. 6 which graphically depicts the relative thermal capacity of air supplied to a space to be conditoned in terms of the volumetric make-up of the supply air. Thus, it can be seen from the graph of FIG. 6, that at point A maximum cooling is required and the total volume of supply air is made up of 80 percent by-pass primary air in addition to the percent constant flow of primary air. As the cooling load in the space decreases, damper 80 modulates as discussed above to restrict the flow of by-pass primary air thereby permitting the commencement of induction of secondary air as discussed above. Thus, at point B on FIG. 6, the thermal'cooling capacity of the supply air is 79 percent of the maximum thermal cooling capacity. In order to accommodate this load, the supply air. volumetrically comprises 20 percent constant flow of primary air, 60 percent by pass primary air and 20 percent secondary air which is induced as recirculated air from the space being conditioned. Continued reduction in the required thermal cooling capacity of the supply air to 20 percent of the maximum thermal cooling capacity (point C on FIG. 6,.) is sensed by the thermostat in the space being conditioned and causes damper 80 to modulate to restrict totally the flow of by-pass primaryair. Thus, at this stage of the operation, the volumetric make-up of the supply air is 20 percent constant flow or primary air and 80 percent secondary air which. is induced as recirculated air from the space being conditioned. I

As the coolingload in the spaceis reduced to the point where no cooling is required to maintain comfort conditions, (D on FIG. 6), .damper 80 is maintained in installations do not require the provision of this additotal flow restricting position with respect to by-pass primary air and damper 96 commences modulation to cause the make-up of secondary air to include both recirculated air from the space being conditioned and air drawnfromthe plenum above suspended ceiling 31. Thus, at this stage of the operation, it can be seen from FIG. 6 that in addition to the 20 percent constant flow of primary air, the remaining volume of secondary air comprises 35 percent recirculated from the conditioned space and 45 percent air drawn from the space above the suspended ceiling.

Finally, as the cooling load in the conditioned space passes to. a negative load or heating load, e.g., .E on FIG. 6, damper 96 continues to be operated to restrict the flow of secondary air drawn as recirculated air from the conditioned space and to further permit the induction of secondary air from the space above the ,sus-

pended ceiling. Thus, at .E where the heating load is 12% of the relative thermal capacity of the system, the total volume of supply air to the system includes in addition to the 20 percent constant flow of primary air,

- percent secondary air induced from the space above the suspended ceiling.

If it is desired to provide additional heating capacity, thereby extending the thermal capacity line beyond .E on FIG. 6, strip heater 99 can be incorporated as discussed above.

It can be seen, therefore, that the terminal unit 35A of FIGS 2-5 embodies an air missing device which combines variable volumes of primary and secondary air to provide a substantially constant volume of supply air to a space at a desired thermal capacity, which capacity is variable in response to variations in the therthe invention is shownin FIGS; 6-8 and designated generally by reference numeral-35B. Unit 35B is similar in basic operation to the embodiment 35A of FIGS. 2-5 with the exception that there is no provision for inducing a flow of air other than as return air from the space being conditioned; V

Referring to FIG. 6, terminal unit 35B comprises a primary air inlet section which is disposed between two induction and mixing sections 122, 123. Induction and mixing sections 122, 123 are in communication with recirculated air plenums. 125, 126 through connector pipes 128, 129, respectively. The structure and operation of induction and mixing sections 122, 123 and recirculated airplenums 125, 126 are identical. Accordingly, the terminal unit 35B is described below in terms of primary air inlet section 120, induction and mixing section 123 and recirculated air plenum 126 with the understanding that the structure and operation of sections 122 and are identical,

Referring therefore to FIG. 7, primary air inlet section 120 is shown to comprise a casing 132 the internal volume of which is divided into a primary air plenum chamber 134 and a primary air by-pass plenum chamber' 135 by a vertically extending partition 137. Depending from casing 132 are mounting strips ,138 which support casing 132 above the Tee bars -l4l of a suspended ceiling.

Plenum chamber 134 is provided with an opening unrestricted communication with a primary air duct 144 which is formed in induction and mixing-section 123. Additionally, partition 137 is provided with a grommeted opening 146 which communicates plenum chamber 134 with by-pass plenum chamber 135. Opening 146 is selectively opened and closed by a throttling device 148 which is identical in structure to the throttling device 78 of the terminal of FIG. 2-5 except that the conical damper 150 of device 148 is coupled directly to the output shaft 151 of damper operator 152 and not indirectly through a spring coupling such as coupling 89. Finally, by-pass plenum chamber 135 is in communication with a primary air space 154 in induc' tion and mixing section 123 through an opening 155 in the casing 132.

Induction and mixing section 123 is best seen in FIG. 8 and comprises a casing 158 which is supported above tioned as is sensed by a thermostat mounted in the space, which controls the operationof damper operator 152. Thus, when a thermostat, e.g. thermostat 106 of FIG. 1 senses such a rise in space temperature as to call for full cooling, damper operator 152 is actuated to withdraw conical damper 150from opening 146 to allow conditioned primary air to flow from' plenum chamber 134 into by-pass plenum chamber 135 through opening 146. Thereafter the primary air flows from by-pass plenum 135 to air space 154 through opening 155 and into mixing chamber 163 through passage 161. The amount of by-pass primary air flowing into chamber 163 under full cooling conditions is such Tee bars 141 by mounting strips 138. The space between mounting strips 138 defines a passage 160 which is in communication with the interior of section 123. In this regard, one of the mounting strips 138 extends into top to define a passage 161 which communicates primary air space 154 with a mixing chamber 163.

Mixing chamber 163 and primary air duct 144 are in communication through aplurality of nozzles 165 which are oriented to' direct air flowing from duct 1 44 downwardly into passage 160 so as to establish a flow of air through passage 160 and into a space to be conditioned through an outlet opening 166. Additionally, the end wall 168 of casing 158 is provided with an opening 169 which places mixing chamber 163 in communication with return air plenum 126 through connector pipe 129.

Recirculated air plenum 126 comprises a casing 171 which is supported above Tee bars 141 by'mounting strips 173 which cooperate to define a passage (not shown) through which air from the conditioned space may be induced for recirculation as secondary air during the operation of terminal unit 35B.

The operation of terminal unit 358 is substantially the same as the-operation of terminal unit 35A. Specifically, primary air is introduced to primary air plenum chamber 134 from a source of primary air such as air pipe 36. A constant volume of the primary air passes unrestrictedly from plenum chamber 134 through opening 142 into air duct 144 and thereafter through nozzles 165 into mixing chamber 163. As noted above, the passage of primary air through nozzles 165-establishes a flow of air through passage 160 and outlet opening 166 into a space to be conditioned. In passage- 160 which defines a mixing duct, the primary air from nozzles 165 is mixed with a volume of additional air from mixing chamber 163 to define the supply air for the space to be conditioned. The additional air is supplied from two sources, viz. by-passed primary air from primary air space 154 through passage 161, and secondary air which in this embodiment comprises recirculated space air from recirculated air plenum '126 through air pipe 129. The relative amounts of additional air from the two sources are established in response to the thermal load in the space to be condithe internal volume of section 123 and is flanged at its when combined with the primary air from nozzles 165,

as to fully satisfy the volumetric requirements of the space to be conditioned. Thus, under full cooling conditions, no airfrom the space is induced from recircu- Iated air plenum 126 through air pipe 129.

As less cooling in' the space is required, conical damper is modulated toward opening 146 by damper operator 152 thus reducing. the flow of by-pass primary air. As the flow of by-pass primary air is throttledjby displacement of conicaldamper 150, and-in order to maintain a substantially constant volumetric flow of supply air into the space to be conditioned, a flow of recirculated air is induced from recirculated air plenum 126in an amount substantially equal to the amount by which the flow of by-pass primary air-is'reduced. Continued reduction in the cooling load is sensed by the thermostat and conical damper l50 con tinues to be modulated by damper control 152'until damper 150 is in sealing'engagement with opening 146. At this stage of the operation, primary air is flowing through nozzles and the flowing air is inducing a flow of secondary air from the conditioned space through recirculated air plenum .126 and air pipe 129.

It can be seen from the foregoing, therefore, that terminal unit 358 enables the provision of a substantially constant volume of supply air to a space to be conditioned, the thermal capacity of which'supply air can be varied over a relatively wide range without the necessity for auxiliary heat exchangers, the provision of supplementary heat exchange media, or other heat exchange equipment as is ordinarily used in known equipment.

A further embodiment of a terminal unit according to the invention is shown in FIGS. 10 and ll and designated generallybythe referenc'ednumeral 35C. Unit 35C operates in the same manner as the unit 35B of FIGS. 68,'and is similar in basic operation to the embodiment 35A of FIGS. 2-5 with the exception that tioned through a central opening in the terminal unit.

Referring therefore to FIGS. 10 and 11, terminal unit 35C can be seen to include a generally square casing 232 having vertically extending walls and a top wall for closing the upper end of the casing. The volume defined-by casing 232 is divided into an upper portion anda lower portion by a partition 234 which is secured to the vertically extending walls of casing 232 and extends tion which cooperates with the inner surface of the vertical walls of casing 232 to define a generally annular channel 235. I

Formed generally centrally in partition 234 is a substantially circular opening 237 which is provided with a grommet 238 around the inner edge thereof.

The upper portion of the inner volume of casing 232 defines a'primary'air plenum chamber 240 which is in communication with a source of primary air to an opening 242 in a vertical wall of the casing 232 which is provided with a collar 244 which may be secured to a suitable air pipe, e.g., air pipe 36 (FIG. 1).

Suspended from the central portion of partition 234 by suitable bolts 250 is a baffle plate 252. Baffle plate '252 separates the lower central portion of the inner volume of casing 232 into a by-pass primary air plenum 254 and a secondary air plenum 256. Additionally, the central portion of baffle'plate 252 is raised to define a mounting surface from which is dependently supported a damper motor 258 which reciprocably operates a generallyconical damper 260 into and out of engage-.

ment with the grommet 238 of opening 237 so as to selectively permit and interrupta flow of primary air from primary air plenum chamber 240 through opening 237 into by-passprimary air plenum chamber 254.

The flow of by-pass primary air through by-pass primary air plenum chamber-.254 is restricted by a first baffle plate 262 which depends from the under-surface of partition 234, and a second baffle plate 264 which is-disposedoutwardly of first baffle plate' 262 and extends upwardly from the upper surface of a plate 252. The outer edge of plate 252 is provided with an angularly downwardly extending flange 266 which-directs the flow of by-pass primary air from bypass plenum chamber 254 outwardly against the surfaces of the vertical walls of casing 232.

Disposed inwardly of the vertical walls of casing 232 is a vertically extending divider 268 which cooperates with the vertically extending walls of casing 232 to define an annular mixing duct 270 which is in communication with a spaceto be conditioned through an outlet opening 272.

Disposed above mixing duct 270 and mounted in transverse partition 234 adjacent the vertically extending walls of casing 232 are a plurality of nozzles 274 which accommodate therethrough a substantially constant flow of primary air from primary air plenum chamber 240 into mixing duct 270.

Secondary air plenum chamber 256 is in communication with the space being conditioned through an open: ing 276 which may be. covered by a suitable ornamental screen or the like. v

The operation of terminal unit 35C is substantially the same as the operation of terminal unit35B. Specifically, primaryair is introduced to primary air plenum chamber 240 from a source of primary air such as air V pipe 36. The constant volume of theprimary air passes unrestrictedly from plenum chamber 240 through nozzles 274 into mixing duct 270. As was discussed above with respect to the other embodiment, the passage of primary air through nozzles 274 establishes a flow of air through mixing duct 270 and outlet opening 272 into a space tobe conditioned. In mixing duct 270, theprimary air from nozzle '274 is mixed with a volume of -ad-' ditional air to define the supply air for the space to be from secondary conditioned. The additional air is s up- I plied selectively from two sources, viz. by-passed primary air from primary air plenum chamber' 240 a through opening 237 and by-pass primary air plenum chamber 254, and secondary air which, in this embodiment, comprises recirculated space air from secondary air plenum 256. I

The relative amounts of additional air from the two sources are established in'response to the thermal load in the space to be conditioned as is sensed'by a thermostat mounted in the space which controls the operation of damper motor 258. Thus, when a thermostat, e.g.,

thermostat 106 of FIG. 1, senses such a rise in space temperature as to call for full cooling, damper motor 258 is actuated to withdraw conical damper 260 from opening 237 to allow conditioned primary air to flow from plenum chamber 240 into by-pass plenum chamber 254. Thereafter, the by-pass primary air flows from by-pass plenum 254 over flange 266 and into mixing duct 270 to augment the flow of primary air from nozzles 274 and to define supply air to'the space to'be conditioned. As was the case with respect to the embodi ment of FIGS. 6-8 above, no air from the space is induced for recirculation fromsecondary air plenum 256 under full cooling conditions As less cooling in the space is required, conical damper 260 is modulated toward opening 237 by damper motor 258, thus reducing the flow of by-pass primary air. As the flow of by-pass primary air is throttled by the displacement of conical damper 260, and in order to maintain a substantially constant volumetric flow of supply air into' the space to be conditioned, a flow of recirculated air' is induced from secondary air plenum 256'in an amount substantially equal to the amount by which the flow of by-pass primary air is reduced. Continued reduction in the cooling load is sensed by the thermostat and conical damper 260 continues to be modulated by damper motor 258 until damper 260 is in sealing engagement with grommet 238 of opening 237. At this stage of the operation, primary air is flowing through nozzles 274 and the flowing air is inducing a flow of secondary air which comprises recirculated air drawn from the conditioned space through secondary air plenum 256.

Terminal unit 35C, therefore, defines an air terminal unit wherein supply air is discharged into a space through an annular peripheral outlet opening and air to berecirculated through the unit as secondary air is drawn into thespace through'a central opening 276. Thus, this embodiment provides for desirable air circulation conditions. I

Still another embodiment of terminal unit according to the invention is shown in FIGS. 12-14 and designated generally by reference numeral 35D. This e'mbodiment is very similar in structure to that disclosed in FIGS. 10 and 11. However, it is adapted for use in an air conditioning system wherein independent supplies of primary air are supplied to each terminal unit. One supply'which shall be designated interior" primary air for purposes of this description is provided unit and thereafter separated so that the air for the pe- 13 rimeter system may be subjected to further temperature central either by reheating at the central conditioning unit or by reheating at the individual terminal units to accomplish individual room control.

Considering'terminal unit 35D in detail, and referring particularly to the figure a generally square casing 332 can be seen to include four vertically extending walls and a top wall for closing in the upper end of the casing. The volume defined by casing 332 is divided into an upper portion for defining an interior primary air ple num chamber 340, an annular perimeter primary air plenum chamber 341, a by-pass primary air plenum 354, a secondary air plenum 356 and an annular mixing duct 370.

Interior primary air is supplied to interior primary air plenum chamber 340 through an air pipe 336 which is connected to a suitable source of interior primary air (not shown). Similarly, annular'perimeter primary air plenum 341 communicates with air pipe 337 through a perimeter primary air inlet chamber 342. Air pipe 337 is connected to a suitable source of perimeter primary air (not shown).

Perimeter primary air plenum 341 and by-pass primary air plenum 354 are separated from interior primary air plenum chamber 340 by a partition 334 which extends transversely of casing 332. except for an-inclined portion 335 on the end thereof adjacent air pipe 336. Formed generally centrally in partition 334 is a substantially circular opening 337 which is provided with a grommet 338 around the inner edge thereof.

Secondary air plenum 356 is separated'from by-pass a plurality of nozzles 374 which accommodate there'- i through a substantially constant flow of perimeter primary air into mixing duct 370.

Secondary air plenum 356 is in communication with v .exception that in terminal unit 35C the constant flow of primary air and bypass primary air are supplied to the unit directly from a singleair pipe source whereas in terminal unit 35D, the constant flow of primary air and the by-pass primary air are provided from'se'parate sources, viz. the perimeter primary air system and the t interior primary air system.

Thus, in operating terminal unit 351); a constant volume of perimeterprimary .air is introduced into champrimary air plenum 354 by a baffle plate 352 which is suspended below partition 334 by suitable bolts 350. Additionally, in the same manner as discussed above with respect to terminal unit C, the central portion of baffle plate 352 is raised to define a mounting surface from which is dependently supported a damper motor 358 which reciprocably operates a generally conical damper 360 into and out of engagement with the grommet 338 of opening 337 so as to selectively permit and interrupt a flow of interior primary air from plenum chamber 340 into by-pass primary air plenum 354. In this regard, the interior primary air flowing from plenum 340 to plenum 3354 functions as by-pass primary air in the context of the term as used with respect to the descriptionof terminal units 35A, 35B and 35C.

The flow of by-pass (interior) primary air through bypass primary air plenum chamber 354 is restricted by a first baffle plate 362 which depends from the undersurface of partition 334, and a second bafile plate 364 which is disposed outwardly of first baffle plate 362 and extends upwardly from the upper surface of baffle plate 352. The outer edge of baffle plate 352 is provided with an angularly downwardly extending flange 366 which directs the flow of by-pass (interior) primary air from by-pass plenum 354 outwardly against the surfaces of the vertical walls of casing 332.

Disposed inwardly of the vertical walls of casing 332 is a vertically extending divider 368 which cooperates 'with the vertically extending walls of casing 332 to define annular mixing duct 370 which is in communication .with a space to be conditioned through an outlet opening 372. 1

Disposed above mixing duct 370 and mounted in the lower transverse walls of perimeter primary air plenum 341 and perimeter primary air inlet chamber 342 are her 342 and plenum 341 from air pipe 337. The constant volume of primary air passes unrestrictedly from chamber 342 and plenum 341 into mixing duct 370. As was discussed above with respect to the other-embodiments, the passage of primary air through nozzles 374 establishes a flow of air through mixing duct 370 and outlet opening 372 into the space to be conditioned. In mixing duct 370, the primary air from nozzles374- is mixed with a volume of additional air to define the supply air for the spaceto be conditioned. The additional air is supplied selectively from two sources, viz. bypassed (interior) primary air from plenum chamber 340 through opening 337 and by-pass primary air plenum chamber 354, and secondary air which, in this embodiment, comprises recirculated space air from secondary air plenum 356.

The relative amounts of additional air from the two sources are established inresponseto the thermal load in the space tobe conditioned as is sensed by a thermostat mounted in the space which controls the operation of damper motor 358. Additionally, the thermostat in the space may be utilized to control the temperature at which perimeter air is introduced tothe unit, e.g. by

controlling a reheater upstream of air pipe 337. Thus,

- interior primary air flows as by-pass primary airfrom by-pass plenum '354, over flange 366 and into mixing duct 370 to augment the flow of primary air from nozzles 374 and to define supply air to the space to be conditioned. As wasthe case with respect to the abovedescribed embodiments, no air from the conditioned.

space is induced for recirculation from secondary air plenum 356 under full cooling conditions.

As less cooling in the space is required, conical damper 360 is modulated toward opening 337' by' damper motor 358, thus reducing the flow of by-pass 5 (interior) primary air. As the flow of by-pass primary air-is throttled by the displacement of conical damper 360, and in order to maintain a substantially constant volumetric flow of supply air into the space to be condi-'- tioned, a flow of recirculated air is induced from secondary air plenum 356 in an amount substantially equal to the amount by which the flow of by-pass primary air is is reduced. Continued reduction in the cooling load is sensed'by the thermostat'and conical damper 360 continues to be modulated by damper motor 358 until damper'360 is in sealing engagement with grommet 338 of opening 337. At this stage of the operation, perimeter primary air is flowing through nozzles 374 and the air thus flowing is inducing a flow of secondary air which comprises recirculated air drawn from the condtiioned space through secondary air plenum 356.

COntinued reduction in the cooling load in the conditioned space will cause the space thermostat to actuate heating meanssuch as a reheater coil for increasing the temperature of 'the perimeter primary air as required by the particular space thermal load.

It should also be recognized that the basic terminal units of the invention may be modified for wall mounting without departing from the basic teaching and without departing from the scope of the method of the invention.

It is considered to be manifest that many modifications and variations can be made to the described embodiments without departing from the scope and teaching of the present invention.

What is claimed is:

I. In a terminal unit for an air conditioning system for controlling the condition of air in a space comprising:

a. casing means, V

b. a continuous end to end mixing duct means through said casing means having an inlet for air to be induced from said space and an outlet for delivering supply air from said casing means into said space to be conditioned,

c. means on said casing means for connecting said terminal unit to a source of conditioned primary air,

d. a primary air plenum in-said casing means in communication with the conditioned primary air delivered to said terminal unit,

e. said primary air plenum having induction means for introducing a constant relatively small proportion of the conditioned primary air into the mixing duct to induce a flow of air from said space into the inlet of said mixing duct .and a flow of supply air from the outlet for said mixing duct into the space to be conditioned, I I

f. by-pass means in said casing means in communication with the conditioned primary air and connected to the mixing duct for delivering a supply of damped conditioned primary air into the mixing air duct at a'point spaced from and independent of the induction air delivered by the means to introduce induction air into the mixing duct, g. damper means connected in said by-pass means including, a perforated means and a reciprocable means to uncover varying portions of said perforated means to effect damping ofthe conditioned primary air and to vary the volumeof conditioned air by-passed through said by-pass means, and h. means responsive to changes in the thermal requirements of the space to be conditioned for actuating said damper means to regulate the conditioned primary air to be delivered by said by-pass means into the mixing duct relative the volume of air induced from the space through the inlet to the mixing duct and the conditioned'primary air for in- 16 v I duction whereby a constant volume of supply air including a ratio of by-passconditioned air and induced space air is delivered by the terminal 'unit to provide the necessary thermal capacity for meeting the cooling requirements of the space to be conditioned.

2. In a terminal unit as claimed in claim 1 wherein,

a; the by-pass means is operatively connected to the primary air plenum at one end. and has an outlet communicating with the mixing duct at the said spaced point, and I b. the means responsive to the condition of the air in the space includes, a normally closed damper .means disposed in the bypass means, and anactuator for moving said damper means asIa function of the variations in the thermal requirements of the space to be conditioned.

3. In a terminal as claimed in claim 2' wherein,

a. said casing means has means forming an opening therein to provide communication between the mixing duct and a source of air outside said termi-' nal unit, and

b. second damper means operatively associated with the means responsive to changes in thermal conditions of the space to be conditioned'and disposed to normally maintain said opening closed whenever conditioned primary air is being by-passed into the mixing duct,

c. said second damper means operable in association with the means responsive to uncover the opening to changes in thermal conditions of the space to be conditioned to actuate the opening means for delivery through said opening of varying volumes of outside air relative to the volume of air induced from the space to modulate the temperature ofthe air from the space and to provide a constant volume of conditioned supply air through said-mixing duct outlet including, a ratio of primary air, outside air, and induced space air to obtain the necessary thermal capacity for meeting the heating requirements of the space to be conditioned.

4. In a terminal unit as claimed in claim 3 wherein said second damper means is positioned in the terminal unit to simultaneously and selectively vary the opening of the inlet forthe mixing air in the casing.

5. In a terminal unit as claimed in claim 3 incuding,

a. meansmounted adjacent said'opening for heating the outisde air which passes throughthe opening into'the mixing duct, and

b. means for controlling delivery of heating media to the heating means.

6. In a terminal unit for an air conditioning system for controlling the condition of air in a space comprising:

' said casing means to said space, c. means in said casing means for connecting said termmal unit to a source of conditioned primary air,

d. a primary air plenum in said casing means in communication with conditioned primary air delivered duct and opening for outside ducing a constant relatively small proportion of the conditioned primary air into the mixing duct to induce a flow of conditioned supply air from said mixing duct through said outlet into the space,- by-pass means in said casing means in communication with the conditioned primary air delivered to the terminal unit and connected to the mixing duct for delivering a supply of damped conditioned primary air into the mixing duct at a point spaced from and independent of the induction of air delivered by the nozzle means, means in said casing providing an opening between said mixing duct and a source of outside air, and g. means responsive to the changes in the thermal re quirements of the space to be conditioned to selectively and alternativerly regulate the relative mixture of conditioned by-pass air, the outside air and air from said space entering the mixing duct whereby a constant volume of conditioned supply airis delivered from the mixing duct through said outlet into the space to meet the thermal requirements for selectively and alternatively cooling and heating said space as may be required. 7. in a terminal unit as claimed in claim 6 including,

a. means mounted adjacent the means forming the opening in the casing means for heatingairpassing therethrough, and b. means to control the operation of said heating means. 8. In a terminal unit as claimed in claim 1 wherein,

a. the mixing duct is U-shaped in cross-section and forms a space on one side of said casing means in communication with the space to be conditioned,

b. the primary air plenum is adjacent the mixing duct to permit the means for introducing induction air to the mixing duct to be disposed medially between the inlet and the outlet for the mixing duct,

c. the by-pass means includes, a by-pass chamber,

and passage means passing on .the side of the primary air plenum remote from the mixing duct, and

d. and passage means having an outlet at the point in the mixing duct spaced from the means for introducing induction air into said'mixing duct.

9. In a terminal unit as claimed in claim 8 wherein the outlet of the passage means is downstreamfrom the means for introducing induction air into said mixing duct. t

10. In a terminal unit as claimed in claim 1 wherein,

d. said by pass means includesra-plurality of spaced by-pass' passages, each 'operatively associated with a given one'of'the spaced passage means of the mixing duct means;

11. In a terminal unit as claimed in claim 10, wherein,

a. the spaced passage means include a T-shaped casing forming a plenum means operatively associated with the outlet for each respective passage,

b. said casing means has a T-shaped central section housing'the portions of the inlet ends of the spaced passage means of the mixingduct means, and the operatively. associated primary air plenums and by-- pass means therefor, and

" c. a single damper means is disposed in said T-shaped central casing common to the plurality of spaced by-pass passages.

12. In a terminal unit as claimed in claim 1, wherein,

a. the mixing duct means is disposed about the perimeter of the casing means,v r

b. said by-pass means includes-anannular convoluted passage disposed inwardly of the primary air plenum, and said convoluted passage has an inlet in communication with said primary air plenum and" an outlet in communication with the mixing duct means,

c. said means responsive to the condition of the air in the space includes a damper disposed centrally in said casing for operative association with the inlet for the annular convoluted passage to main tain said inlet normally closed, and means for actuating the damper to open said-inlet.

13. In a terminalunit as claimed .in 12 wherein the outlet for the annular convoluted passage by the bypass means is disposed to communicate with the spaced point on the mixing duct between the point where the induction air enters the mixing duct and the induced air from the space enters the mixing duct.

14. In a terminal unit as claimed in claim 1, wherein,

in the space includes, a damper disposed centrally in said casing for operative associationwith the inlet for the annular convoluted passage, and normally maintain said inlet closed, and means for actuating the damper to open said inlet.

15. In a terminal unit as claimed in claim 14 wherein the outlet for the convoluted passage of the by-pass means is disposed to communicate with a spaced point in the mixing duct between the point where the induction air enters the mixing duct and the induced air from the space enters the mixing duct.

16. In a terminal unit as claimed in claim 14 'wherein the mixing duct includes,

a. guide vanes, and

b. means forming a structure inwardly of theoutlet for the mixing duct.

l i k 

1. In a terminal unit for an air conditioning system for controlling the condition of air in a space comprising: a. casing means, b. a continuous end to end mixing duct means through said casing means having an inlet for air to be induced from said space and an outlet for delivering supply air from said casing means into said space to be conditioned, c. means on said casing means for connecting said terminal unit to a source of conditioned primary air, d. a primary air plenum in said casing means in communication with the conditioned primary air delivered to said terminal unit, e. said primary air plenum having induction means for introducing a constant relatively small proportion of the conditioned primary air into the mixing duct to induce a flow of air from said space into the inlet of said mixing duct and a flow of supply air from the outlet for said mixing duct into the space to be conditioned, f. by-pass means in said casing means in communication with the conditioned primary air and connected to the mixing duct for delivering a supply of damped conditioned primary air into the mixing air duct at a point spaced from and independent of the induction air delivered by The means to introduce induction air into the mixing duct, g. damper means connected in said by-pass means including, a perforated means and a reciprocable means to uncover varying portions of said perforated means to effect damping of the conditioned primary air and to vary the volume of conditioned air by-passed through said by-pass means, and h. means responsive to changes in the thermal requirements of the space to be conditioned for actuating said damper means to regulate the conditioned primary air to be delivered by said by-pass means into the mixing duct relative the volume of air induced from the space through the inlet to the mixing duct and the conditioned primary air for induction whereby a constant volume of supply air including a ratio of by-pass conditioned air and induced space air is delivered by the terminal unit to provide the necessary thermal capacity for meeting the cooling requirements of the space to be conditioned.
 2. In a terminal unit as claimed in claim 1 wherein, a. the by-pass means is operatively connected to the primary air plenum at one end and has an outlet communicating with the mixing duct at the said spaced point, and b. the means responsive to the condition of the air in the space includes, a normally closed damper means disposed in the bypass means, and an actuator for moving said damper means as a function of the variations in the thermal requirements of the space to be conditioned.
 3. In a terminal as claimed in claim 2 wherein, a. said casing means has means forming an opening therein to provide communication between the mixing duct and a source of air outside said terminal unit, and b. second damper means operatively associated with the means responsive to changes in thermal conditions of the space to be conditioned and disposed to normally maintain said opening closed whenever conditioned primary air is being by-passed into the mixing duct, c. said second damper means operable in association with the means responsive to uncover the opening to changes in thermal conditions of the space to be conditioned to actuate the opening means for delivery through said opening of varying volumes of outside air relative to the volume of air induced from the space to modulate the temperature of the air from the space and to provide a constant volume of conditioned supply air through said mixing duct outlet including, a ratio of primary air, outside air, and induced space air to obtain the necessary thermal capacity for meeting the heating requirements of the space to be conditioned.
 4. In a terminal unit as claimed in claim 3 wherein said second damper means is positioned in the terminal unit to simultaneously and selectively vary the opening of the inlet for the mixing duct and opening for outside air in the casing.
 5. In a terminal unit as claimed in claim 3 incuding, a. means mounted adjacent said opening for heating the outisde air which passes through the opening into the mixing duct, and b. means for controlling delivery of heating media to the heating means.
 6. In a terminal unit for an air conditioning system for controlling the condition of air in a space comprising: a. casing means, b. mixing duct means in said casing means having an inlet for air to be induced from said space and an outlet for delivering conditioned supply air from said casing means to said space, c. means in said casing means for connecting said terminal unit to a source of conditioned primary air, d. a primary air plenum in said casing means in communication with conditioned primary air delivered to the casing means having nozzle means for introducing a constant relatively small proportion of the conditioned primary air into the mixing duct to induce a flow of conditioned supply air from said mixing duct through said outlet into the space, e. by-pass means in said casing means in communication with the conditioned primary air delivered to the terminal unit and connected to the mixing duct for delivering a supply of damped conditioned primary air into the mixing duct at a point spaced from and independent of the induction of air delivered by the nozzle means, f. means in said casing providing an opening between said mixing duct and a source of outside air, and g. means responsive to the changes in the thermal requirements of the space to be conditioned to selectively and alternativerly regulate the relative mixture of conditioned by-pass air, the outside air and air from said space entering the mixing duct whereby a constant volume of conditioned supply air is delivered from the mixing duct through said outlet into the space to meet the thermal requirements for selectively and alternatively cooling and heating said space as may be required.
 7. In a terminal unit as claimed in claim 6 including, a. means mounted adjacent the means forming the opening in the casing means for heating air passing therethrough, and b. means to control the operation of said heating means.
 8. In a terminal unit as claimed in claim 1 wherein, a. the mixing duct is U-shaped in cross-section and forms a space on one side of said casing means in communication with the space to be conditioned, b. the primary air plenum is adjacent the mixing duct to permit the means for introducing induction air to the mixing duct to be disposed medially between the inlet and the outlet for the mixing duct, c. the by-pass means includes, a by-pass chamber, and passage means passing on the side of the primary air plenum remote from the mixing duct, and d. and passage means having an outlet at the point in the mixing duct spaced from the means for introducing induction air into said mixing duct.
 9. In a terminal unit as claimed in claim 8 wherein the outlet of the passage means is downstream from the means for introducing induction air into said mixing duct.
 10. In a terminal unit as claimed in claim 1 wherein, a. said mixing duct means includes a plurality of spaced passage means, b. each of said spaced passage means having an inlet and an outlet in communication with the space to be conditioned, c. said primary air plenum includes a plurality of spaced primary air plenum sections equivalent to the number of spaced passage means, and each of the spaced sections of the primary air plenum have induction means operatively disposed in a given one of said spaced passage means for inducing a flow of air through said passage means, d. said by-pass means includes a plurality of spaced by-pass passages, each operatively associated with a given one of the spaced passage means of the mixing duct means.
 11. In a terminal unit as claimed in claim 10, wherein, a. the spaced passage means include a T-shaped casing forming a plenum means operatively associated with the outlet for each respective passage, b. said casing means has a T-shaped central section housing the portions of the inlet ends of the spaced passage means of the mixing duct means, and the operatively associated primary air plenums and by-pass means therefor, and c. a single damper means is disposed in said T-shaped central casing common to the plurality of spaced by-pass passages.
 12. In a terminal unit as claimed in claim 1, wherein, a. the mixing duct means is disposed about the perimeter of the casing means, b. said by-pass means includes an annular convoluted passage disposed inwardly of the primary air plenum, and said convoluted passage has an inlet in communication with said primary air plenum and an outlet in communication with the mixing duct means, c. said means responsive to the condition of the air in the space includes a damper disposed centrally in said casing for operative association with the inlet for the annular convoluted passage to maintain said inlet normally closed, and means for actuating the damper to open said inlet.
 13. In a terminal unit as claimed in 12 wherein the outlet for the annular convoluted passage by the by-pass means is disposed to communicate with the spaced point on the mixing duct between the point where the induction air enters the mixing duct and the induced air from the space enters the mixing duct.
 14. In a terminal unit as claimed in claim 1, wherein, a. the mixing duct means is disposed about the perimeter of the casing means, b. said by-pass means and said primary air plenums are connected independently of each other to the source of conditioned primary air, c. said by-pass means includes an annular convoluted passage disposed inwardly of the primary air plenum, and said annular convoluted passage has an inlet in communication with the source of conditioned primary air and an outlet in communication with the mixing duct, d. said means responsive to the condition of the air in the space includes, a damper disposed centrally in said casing for operative association with the inlet for the annular convoluted passage, and normally maintain said inlet closed, and means for actuating the damper to open said inlet.
 15. In a terminal unit as claimed in claim 14 wherein the outlet for the convoluted passage of the by-pass means is disposed to communicate with a spaced point in the mixing duct between the point where the induction air enters the mixing duct and the induced air from the space enters the mixing duct.
 16. In a terminal unit as claimed in claim 14 wherein the mixing duct includes, a. guide vanes, and b. means forming a structure inwardly of the outlet for the mixing duct. 